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shell arrays electrodeposited on PET-ITO electrodes
Materials Research Bulletin 48 (2013) 1581–1586
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
ZnO films and nanorod/shell arrays electrodeposited on PET-ITO electrodes Mariana Sima a, Eugeniu Vasile b, Marian Sima a,* a b
National Institute of Materials Physics, P.O. Box MG 7, 077125 Magurele, Romania METAV-CD, 31 CA Rosetti Street, 020015 Bucharest, Romania
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2012 Received in revised form 20 December 2012 Accepted 22 December 2012 Available online 29 December 2012
In this work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. ZnO films with high optical transmittance were prepared from a zinc nitrate solution using a pulsed current technique with a reduced pulse time (3 s). The X-ray diffraction pattern of ZnO film deposited on PET-ITO electrode showed that it has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 0 3]. ZnO nanorods were synthesized on electrochemical seeded substrate in an aqueous solution containing zinc nitrate and hexamethylenetetramine. In order to increase the stability of PET-ITO electrode to electrochemical and chemical stresses during ZnO nanorods deposition the surface of the electrode was treated with a 17 wt% NH4F aqueous solution. Electrochemical stability of PET-ITO electrode was evaluated in a solution containing nitrate ions and hexamethylenetetramine. ZnO nanorod/shell arrays were fabricated using eosin Y as nanostructuring agent. Photoluminescence spectra of ZnO nanorod and ZnO nanorod/shell arrays prepared on the surface of PET-ITO electrode were discussed comparatively. By employing the 1.5 mm-length ZnO nanorod/shell array covered with a Cu2O film a photovoltaic device was fabricated on the PET-ITO substrate. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Electronic materials A. Nanostructures C. Electrochemical measurements
1. Introduction Zinc oxide, as a direct, wide bandgap material, has attracted in the last decade intense experimental and theoretical attention for its potential applications in nanoelectronics and optoelectronics. ZnO thin films are grown by different techniques such as pulsed laser deposition (PLD), magnetron sputtering, MOCVD, spray pyrolysis etc. Electrochemical deposition of ZnO has initially been introduced by Lincot et al. [1–3] and Izaki et al. [4,5] using tin oxide/glass and GaN substrates, respectively. ZnO exhibits a large range of nanostructures [6–11], with morphology depending on preparation conditions. Transparent indium-tin-oxide (ITO) electrodes are widely used as substrates for depositing thin films of electro-active materials such as conducting polymers, semiconductors like ZnO, TiO2, CdTe [12–14], etc. which are part of electrochemical and optoelectronic devices and solar cells. For these devices the chemical and electrochemical stability of the electrodes is very important. ITO films are highly transmissive in the visible spectrum and usually have a low resistivity at room temperature. However these properties are instable during exposure to a wide range of
* Corresponding author. Tel.: +40 0 213690185; fax: +40 0 213690177. E-mail address: msima@infim.ro (M. Sima). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.12.045
aggressive aqueous solutions and organic solvents. On the other hand, the electrical and optical properties of ITO films are severely affected by electrode potential and pH in an electrochemical cell. Thus, a severe degradation of the ITO film occurred during exposure to dichloromethane, 1 M HNO3 and 1 M NaOH solutions or during polarization in either of these media [15]. At cathodic polarization in alkaline media, Sn4+ in the ITO film is reduced to the lower valence or metal state in the form of hydroxides of Sn, which attached to the surface, or of a metallic mirror [16]. A similar instability was observed during cathodic polarization of the ITO electrode in acidic pH range [17]. On visual examination, an opaque film was found to form on the surface. After cathodic polarization in acid media, the composition of the ITO film has changed and its resistance has increased with electrochemical treatment time. However, it was observed that the ITO surface is free from the damages of the kind described above when the cathodic potential is limited to 0.8 V vs. SCE and pH > 6 [17]. ITO coated polyethylene terephthalate (PET) substrates are attractive for flexible electronic and optoelectronic devices because they are lightweight, bendable and inexpensive. However, the electronic devices based on PET-ITO substrate require low temperature processing. On the other hand, ITO film is rigid and brittle and it cracks when it is bent or stretched [18]. Thinner ITO films coated on PET substrate (with higher sheet resistance of 60 V2) possess improved electrical stability under mechanical stress. Because
M. Sima et al. / Materials Research Bulletin 48 (2013) 1581–1586
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Table 1 Chemical composition of the used aqueous solutions. Solution
Composition
1 2 3 4 5 6
80 mM Zn(NO3)2 5 mM Zn(NO3)2 + 5 mM hexamethylenetetramine (C6H12N4) 17 wt% NH4F 5 mM KNO3 + 5 mM hexamethylenetetramine 5 mM Zn(NO3)2 + 5 mM hexamethylenetetramine + 0.32 mM eosin Y 0.4 M CuSO4, 0.3 M lactic acid; pH 10.1 with a NaOH solution
thick ITO films (which have higher electrical conductivity) are more brittle and more porous, their electrical and optical stability is lower during chemical and electrochemical processes. ZnO layers were potentiostatically deposited at 0.85 V/Ag– AgCl on ITO/glass substrate from Zn(NO3)2 (0.08 M) solution (75% H2O, 25% ethanol by volume) at 70 8C [19,20]. ZnO nanorod array was electrodeposited on ITO/glass substrate from a solution of 5 mM ZnCl2 and 0.1 M KCl, at 1.0 V/Ag–AgCl and 78 8C. O2 was continuously bubbled onto the working electrode ITO-coated glass surface [20,21]. A similar solution was used for electrodeposition of ZnO nanorods on PET-ITO substrate at 85 8C. For nuclei generation on the surface of the PET-ITO substrate a potential of 1.3 V/SCE was applied for 10 s. After that, the potential was stabilized at 1.0 V/SCE [12]. In the present work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. We proposed some growth techniques of ZnO nanostructures in order to maintain the stability of PET-ITO electrode (14 V2) during electrochemical and hydrothermal processes. An attempt to fabricate a p-Cu2O/n-ZnO heterojonction diode for photovoltaic device on a PET-ITO support was performed.
2. Experimental detail ZnO films, ZnO nanorods, ZnO–eosin Y shell and Cu2O films were deposited by electrodeposition from aqueous solutions 1, 2, 5 and 6, respectively from the Table 1. The working electrode was a commercial PET-ITO substrate with a sheet resistance of the ITO layer of 14 V2. The substrate was thoroughly cleaned in an ultrasonic bath with isopropanol for 15 min prior to use. The electrochemical cell also contained a platinum foil as auxiliary electrode and an Ag/AgCl electrode in saturated KCl as reference electrode. The electrochemical processes were performed using an Autolab PGSTAT 30 potentiostat digitally controlled by a PC computer. The microstructures of the deposits were imaged by field emission scanning electron microscopy (FESEM), using a FEY Quanta Inspect scanning electron microscope. X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance type X-ray diffractometer, in focusing geometry, equipped with copper target X-ray tube and LynxEye one-dimensional detector. Photoluminescence spectra have been recorded at room temperature using an Edinburgh Instruments FL 920 spectrometer (200–900 nm) equipped with a Xe lamp 450 W and double monocromators; the photoluminescence was excited at 350 nm. The parameters of the solar cell were determined from I–V measurements carried out under standard illumination conditions using an AM1.5 solar simulator (L.O.T.-Oriel GmbH & Co. KG, Model LS0306 with a 300 W Xe-Arc lamp and an AM1.5-Global filter (LSZ189) with the specification: 1sun at 18 cm working distance). Photocurrent–voltage (I–V) measurements were performed using an Autolab PGSTAT 30 Potentiostat/Galvanostat (Eco Chemie). 3. Results and discussion Three different ZnO thin films free of macroscopic defects were prepared on PET-ITO substrates using a pulsed-current deposition
Fig. 1. SEM images of ZnO films prepared on PET-ITO substrate by pulsed-current deposition technique (on time 3 s, off time 3 s) using solution 1 from Table 1, at current densities (mA/cm2): (a) 1, (b) 1.25, (c) 1.4; image (d) shows a cross-sectional FESEM micrograph of ZnO film presented in (a).
M. Sima et al. / Materials Research Bulletin 48 (2013) 1581–1586
Fig. 2. Optical transmittance of ZnO films prepared on PET-ITO substrate by pulsed current deposition at current densities (mA/cm2): (a) 1; (b) 1.25; (c) 1.4.
technique at current densities 1, 1.25, and 1.4 mA/cm2 (on time 3 s and off-time 3 s). The electrochemical processes took place at 70 8C in solution 1 from Table 1. The electric charge was 1.2 C/ cm2 for each ZnO thin film. Fig. 1 shows the surface morphologies of these ZnO films prepared by the electrochemical deposition method. The as-deposited films consist of aggregates of hexagonal plates of ZnO grains with estimated thickness of about 6 nm. It was observed that with current density increasing from 1 to 1.4 mA/ cm2 the crystallite edges became more rounded. The thickness of the ZnO film prepared by pulsed current deposition at current density of 1 mA/cm2 was estimated to be 540 nm (Fig. 1(d)). High optical transmittance of approximately 85% (at wavelength of 600 nm) was obtained for the prepared films (Fig. 2). The X-ray diffraction pattern of the ZnO film deposited on PETITO electrode is shown in the Fig. 3(a). Only (0 0 2), (1 0 1), (1 0 3), (2 0 0) and (1 1 2) peaks of hexagonal ZnO can be seen. Relative intensities of diffraction peaks indicate that there is anisotropy in the distribution of crystallographic directions. In comparison with standard powder diffraction data [22] where the main diffraction peak corresponds to the crystalline planes family of Miller indices (1 0 1), it was found that the relative intensity of (0 0 2) peak is stronger compared to the others; also the relative intensity of diffraction peak at angle 2u = 62.8038 corresponding to crystalline planes family (1 0 3) is higher as in diffraction data [22] mentioned above. This demonstrates that ZnO film has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 0 3]. The electrochemical deposition of ZnO is its electroprecipitation process due to nitrate ions reduction on electrode surface in the presence of Zn2+ ions. As a result of nitrate ions reducing at the cathode are generated hydroxyl ions: NO3 þ 2e þ H2 O ! NO2 þ 2OH
(1)
Zn2+ ions precipitate in the presence of hydroxyl anions and the resulting product is spontaneously dehydrated to ZnO: 2þ
Zn
þ 2OH ! ZnðOHÞ2 ! ZnO # þ H2 O
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Fig. 3. XRD patterns of (a) ZnO film, (b) ZnO nanorod array and (c) ZnO nanorod/ shell samples deposited on PET-ITO substrate. The peaks of the ITO coated PET substrate are shown on the pattern from (d).
the cathode surface during on-time are consumed in the chemical reaction during off-time. PET-ITO electrode was stable during ZnO film electrodeposition. On visual examination, it was not observed to form an opaque film on the electrode surface. The neutral pH of the used solution (solution 1) and the use of a pulsed current technique with a reduced pulse time (3 s) for ZnO deposition determined this behavior of ITO electrode. A ZnO thin film (540 nm) prepared on PET-ITO electrode using a pulsed-current deposition technique at current density 1 mA/ cm2 was used as substrate for deposition of ZnO nanorods. A layer of isolated zinc oxide nanorods embedded in a ZnO matrix that resembles a cobweb was obtained (Fig. 4(a)). The electrochemical process took place in solution 2 from Table 1 at constant current of 0.25 mA/cm2 and temperature of 95 8C. On the other hand, a bundle of 2 mm tall ZnO nanorods have been grown on the electrochemical seeded PET-ITO surface. ZnO seed dots were prepared using solution 1, by pulse plating during 30 s, with a cathodic pulse of current density of 1.4 mA/cm2 applied for 3 s (pulse time), followed by a relaxation time of 3 s corresponding to the current interruption. ZnO nanorod arrays (Fig. 4(b)) have been grown for 2000 s from solution 2, in a hydrothermal–electrochemical process at constant current of 0.25 mA/cm2. However, ITO film became opaque during ZnO nanorods electrodeposition. It is well-known that ZnO nanorods can be prepared in a chemical process [23,24] in aqueous solution with Zn(NO3)2 and C6H12N4 as precursors. Hexamethylenetetramine molecules act like a weak base, which hydrolyzes slowly in the hot aqueous solution and, as a result, increases the pH of the solution and induces ZnO formation. The chemical reactions which take place in the solution 2 from Table 1 at 95 8C [25] are expressed by the following equations: ðCH2 Þ6 N4 þ 6H2 O ! 4NH3 þ 6HCHO
(3)
NH3 þ H2 O ! NH4 þ þ OH
(4)
Zn2þ þ 2OH ! ZnðOHÞ2 $ ZnO þ H2 O
(5)
(2)
In galvanostatic deposition the rate of OH ions formation remains constant during the process. Therefore, the use of the galvanostatic method allows a better control of the precipitation process. Pulsed deposition is superior to fixed current deposition in terms of minimizing of OH ions accumulation on the ITO surface, which could suffer a chemical attack at a higher pH. OH ions formed on
On the other hand, the electrochemical reduction of nitrate ions proceeds by a two-electron pathway with the pH increasing on the electrode surface followed by the precipitation of ZnO nuclei (see the Eqs. (1) and (2)).These changes in pH could cause a chemical attack on PET-ITO substrate.
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Fig. 4. FESEM images of ZnO nanorods prepared in hydrothermal–electrochemical processes on (a) PET-ITO/ZnO film, (b) PET-ITO electrode and (c) PET-ITO electrode treated in solution 3 from Table 1; FESEM image (d) shows a ZnO nanorod/shell array (sample NR-S).
In order to increase the stability of PET-ITO electrode to chemical and electrochemical factors we pretreated its surface with a 17 wt% NH4F aqueous solution (solution 3) at 70 8C for 30 min. The reason of this pretreatment of ITO surface is that the resistance of fluorine doped SnO2 films to these factors is better than that of ITO films. The surface of PET-ITO film undergoes changes during the pretreatment mentioned above. The chronopotentiograms from Fig. 5 show the increase of PET-ITO stability in the solution 4, at 95 8C as a result of this pretreatment. Solutions 2 and 4 differ only in terms of metal ions. In the solution 4, in the absence of Zn2+ ion the electrochemical reduction process of nitrate ion will not be followed by a precipitation process. Thus, the combined effects of
pH and potential to the ITO electrode will be seen. In Fig. 5 it is observed that the potential of pretreated PET-ITO electrode do not change significantly during the experiment, as the current (0.25 mA/cm2) was applied. Also, the transparency of this electrode did not change during this time. Untreated PET-ITO electrode became opaque immediately after the electrochemical process has begun. After about 1070 s the ITO film was broken and potential of PET-ITO electrode increased suddenly as seen on the chronopotentiogram of Fig. 5. The treated ITO electrode permitted the growth of ZnO nanorods (sample NR) without an increase of the ITO film opaqueness. Fig. 4(c) shows SEM image of ZnO nanorods grown on the pretreated PET-ITO surface in the same manner as with
Fig. 5. Chronopotentiograms of the electrodes: (a) PET-ITO and (b) PET-ITO treated for 30 min in the solution 3 from Table 1 at 70 8C. The two chronopotentiograms were recorded employing the solution 4 at 95 8C and a constant density current of 0.25 mA/cm2.
Fig. 6. Photoluminescence spectra at room temperature of ZnO samples deposited on the surface of the PET-ITO electrode treated in solution 3: (a) nanorod array and (b) nanorod/shell array. Photoluminescence spectrum of the treated PET-ITO substrate is shown in the Fig. 6(c).
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Fig. 7. XRD pattern of Cu2O film on PET-ITO substrate. The peaks of the ITO coated PET substrate are shown on the pattern from the Fig. 3(d).
Fig. 9. Current density–voltage curve evaluated for Cu2O/ZnO heterostructure under AM 1.5 illumination.
untreated electrode; the length of ZnO nanorods deposited for 2000 s was up to 1 mm and the nanorods density was higher than for the case of the untreated ITO electrode. In another experiment, we prepared a ZnO nanorod/shell array (sample NR-S) by deposition for 2000 s at 0.25 mA/cm2 a ZnO–eosin Y shell from solution 5 on the surface of the previously prepared ZnO nanorods (NR). The loaded eosin Y molecules were removed by treatment in 0.01 M KOH aqueous solution for 30 min (Fig. 4(d)).In comparison with XRD pattern of ZnO film it was found that samples NR and NRS show only (0 0 2) preferred orientation (Fig. 3(b) and (c)). Fig. 6 compares the photoluminescence spectra of ZnO nanorods (NR) (Fig. 6(a)) and ZnO nanorod/shell array (NR-S) (Fig. 6(b)) prepared on the surface of the PET-ITO electrode treated in the solution 3. In addition, the luminescence spectrum of the treated PET-ITO electrode (Fig. 6(c)) is shown in this figure. This substrate presents a large luminescence band in the spectral region 365–450 nm which restrains the observation of excitonic peak of ZnO nanorod array. The UV near-band emission can be identified at 378 nm (3.28 eV) in the PL spectrum of the sample NR by comparison of the spectra (a) and (c) from the Fig. 6. This spectrum does not show a band attributed to the lattice defects. The sample NR-S exhibits a broad visible light emission that extends from about 465 to 680 nm (Fig. 6(b)) and it is assigned to the oxygen defects [26,27]. The high density of defects evidenced by the photoluminescence spectra for the sample NR-S is most probably a consequence of the loading and extracting processes of eosin Y dye. An attempt to fabricate a p-Cu2O/n-ZnO heterojonction diode for photovoltaic device on a PET-ITO substrate was performed. 1.5 mm-length ZnO nanorod/shell array (sample NR-S) was
covered with an electrodeposited Cu2O film with thickness of around 2 mm. Solution 6 from Table 1 was used for electrodeposition process of the Cu2O film. The best performing heterojunction solar cells were obtained by depositing the Cu2O film at pH 12 [20,28]. Nevertheless, we worked at intermediate pH (10.1) due to instability of PET-ITO film at high pH. Cu2O was deposited potentiostatically at 0.45 V. The X-ray diffraction pattern of Cu2O film deposited on PET-ITO electrode is shown in the Fig. 7. The peaks at 36.58, 42.58, and 61.58 were attributed to the (1 1 1), (2 0 0), and (2 2 0) crystal planes of Cu2O with cubic symmetry, respectively (JCPDS05-0667). As seen in Fig. 7 the orientation of the Cu2O film was [1 1 1], according to literature data for Cu2O film prepared at intermediate pH [28]. Fig. 8(a) shows a cross-sectional FESEM image of Cu2O film (with 1 mm thickness) electrodeposited on the top of ZnO nanorod/ shell array and Fig. 8b is a backscattering mode FESEM image of the fabricated p-Cu2O/n-ZnO heterojunction solar cell. A gold electrode was deposited by evaporation on the top of Cu2O film using a metal mask with 2 mm diameter holes. The size of the hole was the solar cell size. Fabricated device was annealed for 5 h at 100 8C on a hotplate [20]. Fig. 9 shows the current density–voltage curve for the fabricated Cu2O/ZnO heterojunction under an AM 1.5 illumination. This cell based on Cu2O film and ZnO nanorod/shell array deposited on a PET-ITO support exhibited a photovoltaic performance of 0.17% power conversion efficiency (PCE) with an open-circuit voltage (VOC) of 0.161 V, a short-circuit current density (JSC) of 3.83 mA/cm2 and a fill factor (FF) of 28%. The cell performance is
Fig. 8. (a) FESEM image of Cu2O film (with 1 mm thickness) electrodeposited on the top of ZnO nanorod/shell array; (b) backscattering mode FESEM image of the fabricated pCu2O/n-ZnO heterojunction solar cell; a gold electrode is deposited on the top of the Cu2O film.
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quite low for practical applications; probable electrodeposition at pH 10.1 increased the defect density in the Cu2O film. However, the use of such fabricated ZnO nanorod/shell array in a Cu2O/ZnO solar cell is advantageous because it offers a large interface between the components of the heterojunction and could provide a direct path from the point of photogeneration of carriers to the conducting substrate.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusion
[12]
ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. Transparent ZnO films have been electrodeposited on PET-ITO substrates using a pulsed-current deposition technique with a reduced pulse time. ZnO nanorod and nanorod/shell arrays were prepared by combined electrochemical–hydrothermal processes on the surface of PET-ITO electrode pretreated with a 17 wt% NH4F aqueous solution. A solar cell based on Cu2O film and ZnO nanorod/ shell array deposited on a PET-ITO support was fabricated; the power conversion efficiency of the cell was 0.17%.
[13] [14]
Acknowledgment The financial support of Romanian Ministry of Education and Research (Core Program Contract PN09-45) is gratefully acknowledged. References [1] S. Peulon, D. Lincot, Adv. Mater. 8 (1996) 166–170. [2] S. Peulon, D. Lincot, J. Electrochem. Soc. 143 (1998) 864–874.
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Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
ZnO films and nanorod/shell arrays electrodeposited on PET-ITO electrodes Mariana Sima a, Eugeniu Vasile b, Marian Sima a,* a b
National Institute of Materials Physics, P.O. Box MG 7, 077125 Magurele, Romania METAV-CD, 31 CA Rosetti Street, 020015 Bucharest, Romania
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2012 Received in revised form 20 December 2012 Accepted 22 December 2012 Available online 29 December 2012
In this work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. ZnO films with high optical transmittance were prepared from a zinc nitrate solution using a pulsed current technique with a reduced pulse time (3 s). The X-ray diffraction pattern of ZnO film deposited on PET-ITO electrode showed that it has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 0 3]. ZnO nanorods were synthesized on electrochemical seeded substrate in an aqueous solution containing zinc nitrate and hexamethylenetetramine. In order to increase the stability of PET-ITO electrode to electrochemical and chemical stresses during ZnO nanorods deposition the surface of the electrode was treated with a 17 wt% NH4F aqueous solution. Electrochemical stability of PET-ITO electrode was evaluated in a solution containing nitrate ions and hexamethylenetetramine. ZnO nanorod/shell arrays were fabricated using eosin Y as nanostructuring agent. Photoluminescence spectra of ZnO nanorod and ZnO nanorod/shell arrays prepared on the surface of PET-ITO electrode were discussed comparatively. By employing the 1.5 mm-length ZnO nanorod/shell array covered with a Cu2O film a photovoltaic device was fabricated on the PET-ITO substrate. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Electronic materials A. Nanostructures C. Electrochemical measurements
1. Introduction Zinc oxide, as a direct, wide bandgap material, has attracted in the last decade intense experimental and theoretical attention for its potential applications in nanoelectronics and optoelectronics. ZnO thin films are grown by different techniques such as pulsed laser deposition (PLD), magnetron sputtering, MOCVD, spray pyrolysis etc. Electrochemical deposition of ZnO has initially been introduced by Lincot et al. [1–3] and Izaki et al. [4,5] using tin oxide/glass and GaN substrates, respectively. ZnO exhibits a large range of nanostructures [6–11], with morphology depending on preparation conditions. Transparent indium-tin-oxide (ITO) electrodes are widely used as substrates for depositing thin films of electro-active materials such as conducting polymers, semiconductors like ZnO, TiO2, CdTe [12–14], etc. which are part of electrochemical and optoelectronic devices and solar cells. For these devices the chemical and electrochemical stability of the electrodes is very important. ITO films are highly transmissive in the visible spectrum and usually have a low resistivity at room temperature. However these properties are instable during exposure to a wide range of
* Corresponding author. Tel.: +40 0 213690185; fax: +40 0 213690177. E-mail address: msima@infim.ro (M. Sima). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.12.045
aggressive aqueous solutions and organic solvents. On the other hand, the electrical and optical properties of ITO films are severely affected by electrode potential and pH in an electrochemical cell. Thus, a severe degradation of the ITO film occurred during exposure to dichloromethane, 1 M HNO3 and 1 M NaOH solutions or during polarization in either of these media [15]. At cathodic polarization in alkaline media, Sn4+ in the ITO film is reduced to the lower valence or metal state in the form of hydroxides of Sn, which attached to the surface, or of a metallic mirror [16]. A similar instability was observed during cathodic polarization of the ITO electrode in acidic pH range [17]. On visual examination, an opaque film was found to form on the surface. After cathodic polarization in acid media, the composition of the ITO film has changed and its resistance has increased with electrochemical treatment time. However, it was observed that the ITO surface is free from the damages of the kind described above when the cathodic potential is limited to 0.8 V vs. SCE and pH > 6 [17]. ITO coated polyethylene terephthalate (PET) substrates are attractive for flexible electronic and optoelectronic devices because they are lightweight, bendable and inexpensive. However, the electronic devices based on PET-ITO substrate require low temperature processing. On the other hand, ITO film is rigid and brittle and it cracks when it is bent or stretched [18]. Thinner ITO films coated on PET substrate (with higher sheet resistance of 60 V2) possess improved electrical stability under mechanical stress. Because
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Table 1 Chemical composition of the used aqueous solutions. Solution
Composition
1 2 3 4 5 6
80 mM Zn(NO3)2 5 mM Zn(NO3)2 + 5 mM hexamethylenetetramine (C6H12N4) 17 wt% NH4F 5 mM KNO3 + 5 mM hexamethylenetetramine 5 mM Zn(NO3)2 + 5 mM hexamethylenetetramine + 0.32 mM eosin Y 0.4 M CuSO4, 0.3 M lactic acid; pH 10.1 with a NaOH solution
thick ITO films (which have higher electrical conductivity) are more brittle and more porous, their electrical and optical stability is lower during chemical and electrochemical processes. ZnO layers were potentiostatically deposited at 0.85 V/Ag– AgCl on ITO/glass substrate from Zn(NO3)2 (0.08 M) solution (75% H2O, 25% ethanol by volume) at 70 8C [19,20]. ZnO nanorod array was electrodeposited on ITO/glass substrate from a solution of 5 mM ZnCl2 and 0.1 M KCl, at 1.0 V/Ag–AgCl and 78 8C. O2 was continuously bubbled onto the working electrode ITO-coated glass surface [20,21]. A similar solution was used for electrodeposition of ZnO nanorods on PET-ITO substrate at 85 8C. For nuclei generation on the surface of the PET-ITO substrate a potential of 1.3 V/SCE was applied for 10 s. After that, the potential was stabilized at 1.0 V/SCE [12]. In the present work, ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. We proposed some growth techniques of ZnO nanostructures in order to maintain the stability of PET-ITO electrode (14 V2) during electrochemical and hydrothermal processes. An attempt to fabricate a p-Cu2O/n-ZnO heterojonction diode for photovoltaic device on a PET-ITO support was performed.
2. Experimental detail ZnO films, ZnO nanorods, ZnO–eosin Y shell and Cu2O films were deposited by electrodeposition from aqueous solutions 1, 2, 5 and 6, respectively from the Table 1. The working electrode was a commercial PET-ITO substrate with a sheet resistance of the ITO layer of 14 V2. The substrate was thoroughly cleaned in an ultrasonic bath with isopropanol for 15 min prior to use. The electrochemical cell also contained a platinum foil as auxiliary electrode and an Ag/AgCl electrode in saturated KCl as reference electrode. The electrochemical processes were performed using an Autolab PGSTAT 30 potentiostat digitally controlled by a PC computer. The microstructures of the deposits were imaged by field emission scanning electron microscopy (FESEM), using a FEY Quanta Inspect scanning electron microscope. X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advance type X-ray diffractometer, in focusing geometry, equipped with copper target X-ray tube and LynxEye one-dimensional detector. Photoluminescence spectra have been recorded at room temperature using an Edinburgh Instruments FL 920 spectrometer (200–900 nm) equipped with a Xe lamp 450 W and double monocromators; the photoluminescence was excited at 350 nm. The parameters of the solar cell were determined from I–V measurements carried out under standard illumination conditions using an AM1.5 solar simulator (L.O.T.-Oriel GmbH & Co. KG, Model LS0306 with a 300 W Xe-Arc lamp and an AM1.5-Global filter (LSZ189) with the specification: 1sun at 18 cm working distance). Photocurrent–voltage (I–V) measurements were performed using an Autolab PGSTAT 30 Potentiostat/Galvanostat (Eco Chemie). 3. Results and discussion Three different ZnO thin films free of macroscopic defects were prepared on PET-ITO substrates using a pulsed-current deposition
Fig. 1. SEM images of ZnO films prepared on PET-ITO substrate by pulsed-current deposition technique (on time 3 s, off time 3 s) using solution 1 from Table 1, at current densities (mA/cm2): (a) 1, (b) 1.25, (c) 1.4; image (d) shows a cross-sectional FESEM micrograph of ZnO film presented in (a).
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Fig. 2. Optical transmittance of ZnO films prepared on PET-ITO substrate by pulsed current deposition at current densities (mA/cm2): (a) 1; (b) 1.25; (c) 1.4.
technique at current densities 1, 1.25, and 1.4 mA/cm2 (on time 3 s and off-time 3 s). The electrochemical processes took place at 70 8C in solution 1 from Table 1. The electric charge was 1.2 C/ cm2 for each ZnO thin film. Fig. 1 shows the surface morphologies of these ZnO films prepared by the electrochemical deposition method. The as-deposited films consist of aggregates of hexagonal plates of ZnO grains with estimated thickness of about 6 nm. It was observed that with current density increasing from 1 to 1.4 mA/ cm2 the crystallite edges became more rounded. The thickness of the ZnO film prepared by pulsed current deposition at current density of 1 mA/cm2 was estimated to be 540 nm (Fig. 1(d)). High optical transmittance of approximately 85% (at wavelength of 600 nm) was obtained for the prepared films (Fig. 2). The X-ray diffraction pattern of the ZnO film deposited on PETITO electrode is shown in the Fig. 3(a). Only (0 0 2), (1 0 1), (1 0 3), (2 0 0) and (1 1 2) peaks of hexagonal ZnO can be seen. Relative intensities of diffraction peaks indicate that there is anisotropy in the distribution of crystallographic directions. In comparison with standard powder diffraction data [22] where the main diffraction peak corresponds to the crystalline planes family of Miller indices (1 0 1), it was found that the relative intensity of (0 0 2) peak is stronger compared to the others; also the relative intensity of diffraction peak at angle 2u = 62.8038 corresponding to crystalline planes family (1 0 3) is higher as in diffraction data [22] mentioned above. This demonstrates that ZnO film has a polycrystalline structure with preferred orientations in the directions [0 0 2] and [1 0 3]. The electrochemical deposition of ZnO is its electroprecipitation process due to nitrate ions reduction on electrode surface in the presence of Zn2+ ions. As a result of nitrate ions reducing at the cathode are generated hydroxyl ions: NO3 þ 2e þ H2 O ! NO2 þ 2OH
(1)
Zn2+ ions precipitate in the presence of hydroxyl anions and the resulting product is spontaneously dehydrated to ZnO: 2þ
Zn
þ 2OH ! ZnðOHÞ2 ! ZnO # þ H2 O
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Fig. 3. XRD patterns of (a) ZnO film, (b) ZnO nanorod array and (c) ZnO nanorod/ shell samples deposited on PET-ITO substrate. The peaks of the ITO coated PET substrate are shown on the pattern from (d).
the cathode surface during on-time are consumed in the chemical reaction during off-time. PET-ITO electrode was stable during ZnO film electrodeposition. On visual examination, it was not observed to form an opaque film on the electrode surface. The neutral pH of the used solution (solution 1) and the use of a pulsed current technique with a reduced pulse time (3 s) for ZnO deposition determined this behavior of ITO electrode. A ZnO thin film (540 nm) prepared on PET-ITO electrode using a pulsed-current deposition technique at current density 1 mA/ cm2 was used as substrate for deposition of ZnO nanorods. A layer of isolated zinc oxide nanorods embedded in a ZnO matrix that resembles a cobweb was obtained (Fig. 4(a)). The electrochemical process took place in solution 2 from Table 1 at constant current of 0.25 mA/cm2 and temperature of 95 8C. On the other hand, a bundle of 2 mm tall ZnO nanorods have been grown on the electrochemical seeded PET-ITO surface. ZnO seed dots were prepared using solution 1, by pulse plating during 30 s, with a cathodic pulse of current density of 1.4 mA/cm2 applied for 3 s (pulse time), followed by a relaxation time of 3 s corresponding to the current interruption. ZnO nanorod arrays (Fig. 4(b)) have been grown for 2000 s from solution 2, in a hydrothermal–electrochemical process at constant current of 0.25 mA/cm2. However, ITO film became opaque during ZnO nanorods electrodeposition. It is well-known that ZnO nanorods can be prepared in a chemical process [23,24] in aqueous solution with Zn(NO3)2 and C6H12N4 as precursors. Hexamethylenetetramine molecules act like a weak base, which hydrolyzes slowly in the hot aqueous solution and, as a result, increases the pH of the solution and induces ZnO formation. The chemical reactions which take place in the solution 2 from Table 1 at 95 8C [25] are expressed by the following equations: ðCH2 Þ6 N4 þ 6H2 O ! 4NH3 þ 6HCHO
(3)
NH3 þ H2 O ! NH4 þ þ OH
(4)
Zn2þ þ 2OH ! ZnðOHÞ2 $ ZnO þ H2 O
(5)
(2)
In galvanostatic deposition the rate of OH ions formation remains constant during the process. Therefore, the use of the galvanostatic method allows a better control of the precipitation process. Pulsed deposition is superior to fixed current deposition in terms of minimizing of OH ions accumulation on the ITO surface, which could suffer a chemical attack at a higher pH. OH ions formed on
On the other hand, the electrochemical reduction of nitrate ions proceeds by a two-electron pathway with the pH increasing on the electrode surface followed by the precipitation of ZnO nuclei (see the Eqs. (1) and (2)).These changes in pH could cause a chemical attack on PET-ITO substrate.
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Fig. 4. FESEM images of ZnO nanorods prepared in hydrothermal–electrochemical processes on (a) PET-ITO/ZnO film, (b) PET-ITO electrode and (c) PET-ITO electrode treated in solution 3 from Table 1; FESEM image (d) shows a ZnO nanorod/shell array (sample NR-S).
In order to increase the stability of PET-ITO electrode to chemical and electrochemical factors we pretreated its surface with a 17 wt% NH4F aqueous solution (solution 3) at 70 8C for 30 min. The reason of this pretreatment of ITO surface is that the resistance of fluorine doped SnO2 films to these factors is better than that of ITO films. The surface of PET-ITO film undergoes changes during the pretreatment mentioned above. The chronopotentiograms from Fig. 5 show the increase of PET-ITO stability in the solution 4, at 95 8C as a result of this pretreatment. Solutions 2 and 4 differ only in terms of metal ions. In the solution 4, in the absence of Zn2+ ion the electrochemical reduction process of nitrate ion will not be followed by a precipitation process. Thus, the combined effects of
pH and potential to the ITO electrode will be seen. In Fig. 5 it is observed that the potential of pretreated PET-ITO electrode do not change significantly during the experiment, as the current (0.25 mA/cm2) was applied. Also, the transparency of this electrode did not change during this time. Untreated PET-ITO electrode became opaque immediately after the electrochemical process has begun. After about 1070 s the ITO film was broken and potential of PET-ITO electrode increased suddenly as seen on the chronopotentiogram of Fig. 5. The treated ITO electrode permitted the growth of ZnO nanorods (sample NR) without an increase of the ITO film opaqueness. Fig. 4(c) shows SEM image of ZnO nanorods grown on the pretreated PET-ITO surface in the same manner as with
Fig. 5. Chronopotentiograms of the electrodes: (a) PET-ITO and (b) PET-ITO treated for 30 min in the solution 3 from Table 1 at 70 8C. The two chronopotentiograms were recorded employing the solution 4 at 95 8C and a constant density current of 0.25 mA/cm2.
Fig. 6. Photoluminescence spectra at room temperature of ZnO samples deposited on the surface of the PET-ITO electrode treated in solution 3: (a) nanorod array and (b) nanorod/shell array. Photoluminescence spectrum of the treated PET-ITO substrate is shown in the Fig. 6(c).
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Fig. 7. XRD pattern of Cu2O film on PET-ITO substrate. The peaks of the ITO coated PET substrate are shown on the pattern from the Fig. 3(d).
Fig. 9. Current density–voltage curve evaluated for Cu2O/ZnO heterostructure under AM 1.5 illumination.
untreated electrode; the length of ZnO nanorods deposited for 2000 s was up to 1 mm and the nanorods density was higher than for the case of the untreated ITO electrode. In another experiment, we prepared a ZnO nanorod/shell array (sample NR-S) by deposition for 2000 s at 0.25 mA/cm2 a ZnO–eosin Y shell from solution 5 on the surface of the previously prepared ZnO nanorods (NR). The loaded eosin Y molecules were removed by treatment in 0.01 M KOH aqueous solution for 30 min (Fig. 4(d)).In comparison with XRD pattern of ZnO film it was found that samples NR and NRS show only (0 0 2) preferred orientation (Fig. 3(b) and (c)). Fig. 6 compares the photoluminescence spectra of ZnO nanorods (NR) (Fig. 6(a)) and ZnO nanorod/shell array (NR-S) (Fig. 6(b)) prepared on the surface of the PET-ITO electrode treated in the solution 3. In addition, the luminescence spectrum of the treated PET-ITO electrode (Fig. 6(c)) is shown in this figure. This substrate presents a large luminescence band in the spectral region 365–450 nm which restrains the observation of excitonic peak of ZnO nanorod array. The UV near-band emission can be identified at 378 nm (3.28 eV) in the PL spectrum of the sample NR by comparison of the spectra (a) and (c) from the Fig. 6. This spectrum does not show a band attributed to the lattice defects. The sample NR-S exhibits a broad visible light emission that extends from about 465 to 680 nm (Fig. 6(b)) and it is assigned to the oxygen defects [26,27]. The high density of defects evidenced by the photoluminescence spectra for the sample NR-S is most probably a consequence of the loading and extracting processes of eosin Y dye. An attempt to fabricate a p-Cu2O/n-ZnO heterojonction diode for photovoltaic device on a PET-ITO substrate was performed. 1.5 mm-length ZnO nanorod/shell array (sample NR-S) was
covered with an electrodeposited Cu2O film with thickness of around 2 mm. Solution 6 from Table 1 was used for electrodeposition process of the Cu2O film. The best performing heterojunction solar cells were obtained by depositing the Cu2O film at pH 12 [20,28]. Nevertheless, we worked at intermediate pH (10.1) due to instability of PET-ITO film at high pH. Cu2O was deposited potentiostatically at 0.45 V. The X-ray diffraction pattern of Cu2O film deposited on PET-ITO electrode is shown in the Fig. 7. The peaks at 36.58, 42.58, and 61.58 were attributed to the (1 1 1), (2 0 0), and (2 2 0) crystal planes of Cu2O with cubic symmetry, respectively (JCPDS05-0667). As seen in Fig. 7 the orientation of the Cu2O film was [1 1 1], according to literature data for Cu2O film prepared at intermediate pH [28]. Fig. 8(a) shows a cross-sectional FESEM image of Cu2O film (with 1 mm thickness) electrodeposited on the top of ZnO nanorod/ shell array and Fig. 8b is a backscattering mode FESEM image of the fabricated p-Cu2O/n-ZnO heterojunction solar cell. A gold electrode was deposited by evaporation on the top of Cu2O film using a metal mask with 2 mm diameter holes. The size of the hole was the solar cell size. Fabricated device was annealed for 5 h at 100 8C on a hotplate [20]. Fig. 9 shows the current density–voltage curve for the fabricated Cu2O/ZnO heterojunction under an AM 1.5 illumination. This cell based on Cu2O film and ZnO nanorod/shell array deposited on a PET-ITO support exhibited a photovoltaic performance of 0.17% power conversion efficiency (PCE) with an open-circuit voltage (VOC) of 0.161 V, a short-circuit current density (JSC) of 3.83 mA/cm2 and a fill factor (FF) of 28%. The cell performance is
Fig. 8. (a) FESEM image of Cu2O film (with 1 mm thickness) electrodeposited on the top of ZnO nanorod/shell array; (b) backscattering mode FESEM image of the fabricated pCu2O/n-ZnO heterojunction solar cell; a gold electrode is deposited on the top of the Cu2O film.
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quite low for practical applications; probable electrodeposition at pH 10.1 increased the defect density in the Cu2O film. However, the use of such fabricated ZnO nanorod/shell array in a Cu2O/ZnO solar cell is advantageous because it offers a large interface between the components of the heterojunction and could provide a direct path from the point of photogeneration of carriers to the conducting substrate.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusion
[12]
ZnO films, nanorod and nanorod/shell arrays were synthesized on the surface of PET-ITO electrodes by electrochemical methods. Transparent ZnO films have been electrodeposited on PET-ITO substrates using a pulsed-current deposition technique with a reduced pulse time. ZnO nanorod and nanorod/shell arrays were prepared by combined electrochemical–hydrothermal processes on the surface of PET-ITO electrode pretreated with a 17 wt% NH4F aqueous solution. A solar cell based on Cu2O film and ZnO nanorod/ shell array deposited on a PET-ITO support was fabricated; the power conversion efficiency of the cell was 0.17%.
[13] [14]
Acknowledgment The financial support of Romanian Ministry of Education and Research (Core Program Contract PN09-45) is gratefully acknowledged. References [1] S. Peulon, D. Lincot, Adv. Mater. 8 (1996) 166–170. [2] S. Peulon, D. Lincot, J. Electrochem. Soc. 143 (1998) 864–874.
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